This invention relates to an on/off power detector in general and, in particular, to a circuit for indicating when a power switch has been opened to a low power dissipation device. In conventional high power type circuits, voltage present at the output side of a power switch tends to drop quickly when the switch is open due to the load connected to the switch.
However, when extremely low power circuitry, such as CMOS digital circuitry, is being powered from a circuit having capacitive elements a problem exists in determining when a switch has been turned off. Power stored in such capacitive elements can power circuits for a relatively long time period before the voltage is substantially reduced.
Determining when a power switch has been opened becomes particularly significant in some applications, such as portable paging equipment, where it is desirable that the circuit perform certain functions whenever a power switch is closed to turn on the unit. Such functions can include providing an audio alert to indicate the presence of sufficient battery voltage to operate the unit as well as to assure that the digital decoding circuitry and/or microprocessor are reset so that any required initial power up logic functions can occur. Where a switch is turned off for only a short period of time and then turned back on, any reset which depends on the device detecting a low voltage at the output of the switch will not operate properly if the switched voltage has not fallen to a sufficiently low level.
Some portable paging receivers utilize a single cell battery and a DC to DC converter to provide stepped up DC operating voltages for powering various circuits. In these devices, the DC to DC converter includes a substantial output capacitor which can provide sufficient output voltage to power a low power circuit for a substantial time, making it impossible to determine the state of the power switch from the voltage level appearing at the output of the converter.
One approach to determining when a switch has been opened is to directly measure the voltage at the output of the power switch, however, this approach offers some serious limitations in the case of battery powered equipment. Because a battery supply can have a normal operating voltage varying over a substantial range and still power the device, a trigger point for such a voltage detector would have to be below the normal battery voltage operating range. Consequently, such a circuit would not detect the opening of a power switch until such time as the voltage has dropped from a substantially higher battery voltage to a trigger point below the lower battery voltage operating range.